Preheat the oven - Set your oven to 400°F (200°C) and line a sheet pan with foil or parchment for easy cleanup.
Make the sauce - In a saucepan, stir together soy sauce, honey, brown sugar, rice vinegar, sesame oil, garlic, ginger, and pineapple juice. Bring to a gentle simmer. Stir in the cornstarch slurry and cook until thickened, then remove from heat.
Season the chicken - Toss the chicken chunks with olive oil, salt, pepper, and garlic powder. Let them hang out for 10 minutes while you prep everything else.
Assemble the sheet pan - Spread the chicken, pineapple, bell peppers, and red onion evenly on the prepared sheet pan.
Add the sauce - Drizzle half the teriyaki sauce over everything and toss gently to coat.
Bake - Roast for 20–25 minutes, stirring halfway through, until the chicken is cooked through and the veggies are just starting to char at the edges.
Optional broil - Want caramelized goodness? Turn on the broiler for 2–4 minutes to crisp up the pineapple and give the chicken those golden, sticky edges.
Finish and serve - Drizzle with the rest of the sauce, sprinkle with sesame seeds and green onions, and serve hot.